1. Field of the Invention
The present invention relates to an ink jet recording apparatus, an ink jet recording head and an ink jet recording method of the serial type in which the image quality and the throughput are improved.
2. Related Background Art
The arts examined by the present inventors to achieve this invention will be described below. (Color-specific heads arranged in scan direction)
FIG. 11 shows a conventional ink jet color printer of the serial type. A print head 1 has a plurality of nozzle columns, and is a device for recording an image on the recording medium by discharging ink droplets to form dots thereon. Different color inks are discharged through different print heads, and by the color mixture of such ink droplets, a color image is formed on the recording medium.
Print data is transmitted via a cable 9 from an electric circuit of a printer itself to print heads. A row of print heads 1K (black), 1C (cyan), 1M (magenta) and 1Y (yellow) are mounted on a carriage 201 to discharge the inks in this order during one scan. For example, when red (R) is formed, magenta (M) ink is first shot onto the recording medium, and then yellow (Y) ink is shot onto a dot of M, so that a dot of red appears.
Similarly, when green (G) is formed, C and Y are shot in this order, and when blue (B) is formed, C and M are shot in this order. However, for example, when G is printed in solid, C is printed, then carriage is caused to scan by the distance of 2.times.P1, and Y is then printed, since print heads are spaced at a certain interval (P1). That is, solid Y is printed on solid C.
This carriage 201 is controlled for movement in a main scan direction by a speed sensing means 5 which senses the scanning speed of carriage and the print position. The power source for this purpose is a carriage drive motor 8, the power of which is transmitted by belts 6, 7 for the sliding movement along a shaft. During the movement in the main scan direction, the printing in a column direction is performed. There are two kinds of print operations in the column direction; one-way printing and two-way printing. Normally, the one-way printing is performed only when carriage is moved in a direction away from a home position (forward direction), but not when carriage is returned to the home position (backward direction). That is, high precision printing is allowed. The term herein used "home position" (hereinafter abbreviated as HP) refers to a position at which carriage 201 is placed opposite a recovery unit 400.
On the other hand, the two-way printing involves a print operation in both forward and backward directions. Hence, high speed printing is allowed. However, since the shooting order of color inks onto the recording medium will be reversed between the forward and backward printing with the arrangement of print heads of this example, the color tint of mixed colors may be different for each scan. In the case of G, it is printed in the order of C and Y in the forward printing, but it is printed in the reverse order of Y and C in the backward printing, resulting in green of a yellow tint, as compared with the printing in the order of C and Y. In this way, normally, the two-way color printing is not allowed due to the occurrence of differences in color tint, and thus can be performed only in the case of monochrome printing. Recovery unit 400 has a function of retaining the state of print heads excellent at all times, wherein in the non-print state, a row of caps enclose the discharge face of print heads to prevent drying.
The function of the recovery unit during the print operation will be described below. In the actual print operation, all the nozzles within one head are not necessarily used. Also, despite plural color print heads provided, some heads may be unused to which print data is not transferred. In this way, if any ink is not discharged successively for a certain period of time (with the print head not capped) during the scanning of carriage, it is apprehended that the discharge performance of ink may decrease due to fixing or drying of the ink on the surface of print head, resulting in a degraded image. To prevent this phenomenon, the print head is forced to discharge the ink through the nozzles within the head at certain intervals, rather than printing data, to hold the state on the surface of print head optimum at any time. This operation is referred to as predischarge.
The ink discharged by this predischarge is directed into caps 420 within the recovery unit 400, and sucked by a recovery pump, not shown, to be stored in a waste ink tank, so that the ink may not splash over the recording medium or inside the printer to produce contamination. Hence, when predischarge is performed during the print operation, it is necessary, in either of the one-way and two-way printing, that carriage 201 returns to HP and placed opposite the row of caps 420. The feeding of recording medium in the sub-scan direction is effected by a sheet feed member (e.g., rubber roller) driven by a sheet feed motor, not shown. A sheet is supplied from a direction of the arrow A as shown in the same figure, and if the sheet arrives at the print position, the print operation is performed by the row of print heads. Thereafter, the sheet is exhausted in a direction of the arrow B by means of sheet exhaust mechanisms 2, 3. Also, the supply of ink is performed from an ink cassette 10K, 10C, 10M, 10Y to a print head for each color.
(Fine recording)
Unlike printing only the character as the monochrome printer, the printing of color image requires various factors such as coloration, gradation and uniformity. In particular, regarding the uniformity, a slight dispersion of each nozzle yielded during the manufacturing process of print head may affect the amount or direction of discharging the ink from each nozzle, in printing, finally causing density unevenness in the printed image and resulting in a degraded image quality.
A specific example of density unevenness will be described using an example of a monochrome print head, for simplification, with reference to FIGS. 12A to 13C. In FIG. 12A, 91 is a print head, which is comprised of eight multi-nozzles in this case, for simplification. 93 is an ink droplet discharged through a multi-nozzle 92, in which it is usually ideal that the ink is discharged with the equal discharge amount and in the same direction as shown in this figure. If such discharge is performed, dots of equal size will be shot on the sheet, as shown in FIG. 12B, so that a complete image without density unevenness as a whole can be obtained (FIG. 12C).
However, in practice, each of nozzles has a dispersion, as previously described, and if directly printing in the above manner, there will occur some dispersion in the size and direction of an ink droplet discharged from each of the nozzles, as shown in FIG. 13A, so that the ink is shot on the sheet, as shown in FIG. 13B. In this figure, blank portions periodically took place which could not meet the area factor of 100% in the main scan direction of head, or conversely, overlapping dots more than necessary, or white streaks as seen in the central part of the figure took place. A set of dots shot in such a state has a density distribution as shown in FIG. 13C in a direction of nozzle column, with the result that such a phenomenon can be normally sensed as a density unevenness simply by seeing with the human eyes. Also, conspicuous streaks may be caused by the dispersion in the sheet feed amount. Thus, a measure against density unevenness or tie streaks for the monochrome ink jet recording apparatuses has been devised in Japanese Laid-Open Patent Application No. 60-107975. This measure will be described briefly with reference to FIGS. 14A to 15C. With this measure, the print head 91 is scanned by three times to complete a print area as shown in FIGS. 12A to 13C, but the one-half area in units of four pixels is completed by two passes (main scan by two times). In this case, the print head of eight nozzles is divided into the upper part of four nozzles and the lower part of four nozzles, wherein dots which one nozzle prints by one time of scan are thinned out to about one-half thereof in accordance with a predetermined array of image data. And with the second scan, remaining image data is embedded with dots to complete the print for the area in units of four pixels. The manner of recording as above described is referred to as a fine recording mode.
With this recording method, the influence inherent to each nozzle on printed image can be reduced even by using a print head identical to that as shown in FIGS. 13A to 13C, resulting in a printed image as shown in FIG. 14B without conspicuous black or white streaks as seen in FIGS. 13A to 13C. Accordingly, density unevenness can be relieved appreciably as shown in FIG. 14C, as compared with that as shown in FIGS. 13A to 13C. In such recording, the image data is divided between the first and second scans to make up for each other in a predetermined array, wherein this image data array (thinning pattern) is typically a cross lattice where each pixel in longitudinal and horizontal directions is crossed, as shown in FIGS. 15A to 15C.
Accordingly, unit print area (herein, in units of four pixels) is completed by the first scan for printing the cross lattice and the second scan for printing the counter-cross lattice. FIGS. 15A, 15B and 15C are views for explaining how the printing for a certain area is formed in such cross and counter-cross patterns, using a print head of eight nozzles, as in FIGS. 12A to 14C. First, at the first scan, the cross pattern is printed using the lower four nozzles (FIG. 15A). Next, at the second scan, the counter-cross pattern is printed after feeding the sheet by the amount of four pixels (one-half all the nozzles) (FIG. 15B). Further, at the third scan, the cross pattern is printed again after feeding the sheet by the amount of four pixels as well (one-half all the nozzles) (FIG. 15C).
In this manner, the print area in units of four pixels is completed for every scan in such a manner as to feed the sheet by the amount of four pixels and alternately perform of the printing of the cross or counter-cross pattern. As described above, a high quality image without density unevenness can be obtained by completing the printing on the same area with different types of nozzles.
&lt;Color-sequential heads arranged in sub-scan direction&gt;
Apart from the print head arranged in the main scan direction, as previously described, there is a print head 301 arranged for each color in a recording medium conveying direction (sub-scan direction). As shown in FIG. 16, an ink discharge portion for each color is arranged along the recording medium conveying direction. One or more ink discharge orifices are arranged in the ink discharge portion for each color. In this figure, the color image forming sequence is K, C, M and Y, but practically, another sequence, for example, Y, M, C, K, may be possible for the print head. The way of forming an image wherein a print head for each color is separately arranged in the recording medium conveying direction, or the sequence of forming the color, is shown in FIG. 17.
The same figure shows the color mixture of K and C, wherein print head 301 forms an image in such a manner as to feed one line for each scan. As will be clear from the figure, K ink is first shot at all times, and then C ink is shot, whereby there occurs no difference in the color tint produced by the change in the order of shooting onto the recording medium. Each scan of the same figure is consistent for both the one-way and two-way printing. In this way, since each color ink is shot onto the recording medium in the same sequence, the two-way printing is allowed, unlike the color-specific head arranged in the main scan direction as previously described, so that the high speed printing can be implemented. To effect the fine printing with this method, the width of line feed is set to be shorter than the width of print head for each color. A pixel is not formed by the same nozzle in the raster direction but formed by using different nozzles with the line feed. FIG. 18 shows an instance wherein the width of line feed is one-half that of print head for each color. As shown in the same figure, the reciprocative printing is possible, but the formation of one line of color image requires the scanning of print head to be performed by eleven times.
However, in the case of the color-specific head arranged in the main scan direction, as previously described, there was a problem that the reciprocative color printing was usually impossible, so that the time required to output the color image was significantly larger. Also, in the case of the color-sequential head arranged in the sub-scan direction, there was a problem that the reciprocative color printing was possible, with the improved throughput attained, but to effect the fine printing mode capable of outputting the higher quality image, a greater number of times of scanning were required corresponding to the number of print heads arranged longitudinally.